A thermodynamic system with zero heat exchange and work is an isolated system. For these systems, the internal energy remains constant.

In the case of a non-isolated system, the change in the internal energy is zero only if the process is cyclic. A thermodynamic process is considered cyclic if the system undergoes a series of changes and returns to its initial state.

Consider a cyclic process that returns to its initial state, undergoing a four-step process. The heat transfer along each path is 40 J, −80 J, −20 J, and 100 J. What is the net work done for this process?

The known quantities are the heat transfer for each step. The unknown quantity is the net amount of work done for this cyclic process.

According to the first law of thermodynamics, the change in the internal energy of the system is the difference between the net heat transfer into the system and the net work done by the system. Since the change in internal energy is zero, the net amount of work done equals the net heat transfer. The net heat transfer is the sum of the heat transfer for each path, equal to 40 J. Hence, the net work done is 40 J.

The net positive work done implies that the work done by the system is greater than the work done on the system. The heat energy supplied to the system is used by the system in doing the external work.